1. Field of the Invention
This invention relates to the providing of drop current to the subscribers served by a remote terminal of a telecommunications system and more particularly to the control of the subscriber drop current with both load and loop length in a system where the remote terminal is powered from the system's central office.
2. Description of the Prior Art
In many telecommunication systems such as multi-channel analog or digital subscriber carrier systems or remote switches or concentrators for example a multiplicity of subscribers located at one or more terminals remote from the central office are serviced by the system. Each of the subscribers have their telephones connected to the associated terminal through a DC loop. The terminal places a predetermined voltage across the subscriber's telephone. When the telephone goes off hook a predetermined current will flow in the loop connecting the telephone to the terminal. A voltage drop will occur in that loop which will depend upon the resistance of the telephone which is substantially fixed and the length of the loop. Therefore the voltage drop in a long loop, that is a telephone located far from the terminal will be substantially larger than the voltage drop in a short loop.
As more of the subscribers serviced by the terminal come off hook an increasing load is placed upon the power circuitry located in that terminal. The terminal must maintain the predetermined voltage and also provide the predetermined current to each of the subscribers. Where the system is of the type wherein the remote terminals are connected to a local source of ac power the meeting of the power requirements for increased load is not difficult. On the other hand the system may be of the type wherein the power for the remote terminals is obtained from the central office. One such central office powered multichannel digital subscriber carrier system is described in copending U.S. patent application Ser. No. 234,894 entitled "A Central Office Powered Digital Subscriber Carrier System" based on the inventions of Gerald Beene et al and filed of even date herewith. In such central office powered systems the amount of power available at the remote terminal is limited. As more of the subscribers connected to that remote terminal come off hook an increased load is placed on the terminal. This load causes the voltage at the input to the terminal to decrease.
The remote terminal generates from the power supppled by the central office the necessary voltages to run the electronics at the terminal and also the predetermined voltage to be placed across each subscriber loop. As the load at the terminal increases and the voltage at the input to the terminal decreases, the power supply at the terminal will find it increasingly more difficult to maintain these voltages. If as a result of decreasing input voltage at the terminal the power supply located therein decreases the voltages it generates the terminal electronics may not operate properly. Improper operation of the electronics could cause the terminal to malfunction and terminate service to the subscribers thereto. In this manner service to the subscribers connected to the terminal would be impaired.
It was then recognized that it would be far more desirable in a central office powered system to maintain the input voltage to the terminal at some minimum acceptable level as more of the subscribers connected to that terminal came off hook. It was further recognized that as conventional drop circuitry operates at a constant voltage and a constant current independent of the length of the loop thereby resulting in a substantial waste of power particularly on short loops it would also be desirable to provide drop circuitry which substantially eliminates this waste of power. Until the present invention no such technique or circuitry was known which would produce these results.
The drop circuitry of the present invention provides for a drop current to the subscriber which is generated by a circuit whose operation is controlled as a function of the length of the loop connecting the subscriber to the terminal. Controlling the generation of the drop current in this manner allows the limited power available at the terminal to be used efficiently with a minimum waste of power. Also included in the drop circuitry is a circuit for decreasing the drop current to all of the off hook subscribers equally whenever the input voltage to the terminal falls below the minimum acceptable level. This level is typically set at that voltage which will insure maximum power transfer from the cable pairs, which connect the terminal to the central office, to the terminal.